Social media is used largely by campaigns to gather and categorize the views of a particular demographic. My guess is that his campaign was interested in your political views and may have followed you to get more information about how people connected to you feel about Trump’s campaign. /twocents

Ref: https://www.washingtonpost.com/politics/cruz-campaign-credits-psychological-data-and-analytics-for-its-rising-success/2015/12/13/4cb0baf8-9dc5-11e5-bce4-708fe33e3288_story.html